Their countries , their laws. It really depends on your definition for not supported too . If we speak about banning then as far as I know I don't think there is a country that actually banned bitcoin , at least nothing official and If there is then Its related to the banking system with one way or another . If its about the lack of usability in ATMs , local shops then there are plenty of countries and It takes time as some countries have more priorities .
Some countries are not supported Bitcoin because in Bitcoin system, many banking system hampers by Bitcoin online business. Bank didn't get any interest or tax through Bitcoin. The government does not get any tax from Bitcoin. So that many countries are not supported it.
In contradict with statement above that there is no country actually ban bitcoin. But actually they exist like China, Taiwan, Russia, Germany, Bolivia are the few list of countries that prohibit the use of bitcoin for public. One of the most common reason is the the threat of one's central bank and to avoid any illegalities and abusive use of virtual currency.